A vehicle steering system is required to provide a steering reaction that reflects the vehicle condition and road condition for the vehicle operator to operate the vehicle in a comfortable way. The electric power steering system is increasingly widely used in current automobiles, and is often equipped with a control system that provides a reactive torque to the steering wheel according to various parameters such as the steering angle, yaw rate, vehicle speed and so on at the same time as providing an assisting steering torque.
According to a known vehicle steering system, when a disturbance such as a side wind is applied to a vehicle, an assisting steering torque that counteracts the disturbance is produced. This system comprises a manual steering means for allowing the steerable road wheels to be manually steered, an electric motor for applying a assisting steering torque to the steerable road wheel and a control means for adjusting the assisting steering torque produced by the electric motor according to an output of a vehicle condition detecting means. The control means detects changes in the output values of the vehicle condition detecting means for each unit time, and actuates the electric motor so as to cancel the changes. See Japanese patent No. 3110891 (patent document 1).
In case of an electric motor vehicle such as a pure electric motor vehicle and a hybrid vehicle using both an electric motor and an internal combustion engine, the rear wheels may be connected to an electric motor, and the electric motor may be used as a regenerative brake. However, when the regenerative brake is activated, it may affect the behavior of the vehicle to such an extent that the vehicle operator may experience an unfamiliar feeling or even a discomfort. This problem may be particularly acute when the regenerative brake is applied only to the rear wheels without using a disk brake or other mechanical brake to brake the front wheels. When the rear wheels are braked without braking the front wheels, the vehicle temporarily acquires an oversteer tendency. A steering system incorporated with a means for canceling external disturbances may detect such a behavior of the vehicle as a disturbance, and may be able to control the vehicle behavior. However, as the steering system would be able to detect the disturbance only after the yaw rate and other conditions of the vehicle have changed as a result of the actuation of the regenerative brake, the response of the steering system is necessarily delayed, and the unfamiliar response of the vehicle may not be adequately controlled.